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FIGURE 10.16 The mole concept for (A) elements, (B) compounds, and (C) molecular substances. A mole contains 6.02 × 10 par-
ticles. Since every mole contains the same number of particles, the ratio of the mass of any two moles is the same as the ratio of the masses
of individual particles making up the two moles.

QUANTITATIVE USES OF EQUATIONS molecules of ammonia. Since 6.02 × 10 molecules is the num-
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A balanced chemical equation can be interpreted in terms of ber of particles in 1 mole, the coefficients therefore represent
(1) a molecular ratio of the reactants and products, (2) a mole ratio the numbers of moles involved in the reaction. Thus, 3 moles of
of the reactants and products, or (3) a mass ratio of the reactants H 2 react with 1 mole of N 2 to produce 2 moles of NH 3 .
and products. Consider, for example, the balanced equation for The mole ratio of a balanced chemical equation leads to
reacting hydrogen with nitrogen to produce ammonia. the concept of a mass ratio interpretation of a chemical equa-
tion. The gram-formula weight of a compound is the mass in
3 H 2 ( g) + N 2 ( g) → 2 NH 3 ( g)
grams of 1 mole that is numerically equal to its formula weight.
From a molecular point of view, the equation says that Therefore, the equation also describes the mass ratios of the
3 molecules of hydrogen combine with 1 molecule of N 2 to form reactants and the products. The mass ratio can be calculated
2 molecules of NH 3 . The coefficients of 3:1 → 2 thus express a from the mole relationship described in the equation. The three
molecular ratio of the reactants and the products. interpretations are summarized in Table 10.1.
The molecular ratio leads to the concept of a mole ratio Thus, the coefficients in a balanced equation can be inter-
since any number of molecules can react as long as they are in preted in terms of molecules, which leads to an interpretation
the ratio of 3:1 → 2. The number could be Avogadro’s num- of moles, mass, or any formula unit. The mole concept thus
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ber, so (3) × (6.02 × 10 ) molecules of H 2 will combine with provides the basis for calculations about the quantities of reac-
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(1) × (6.02 × 10 ) molecules of N 2 to form (2) × (6.02 × 10 ) tants and products in a chemical reaction.
266 CHAPTER 10 Chemical Reactions 10-16